Patent GB-1394075-A (GB)
Title: Extendable mult'plexer
IPC: H04J3/10
Filed: 1972-06-21
Abstract: 1394075 Transmission DDI COMMUNICATIONS Inc 21 June 1972 [30 June 1971] 28988/72 Heading G4H In a multiplexing system, the transmitter or receiver is extendible. At a transmitter, a power supply module, a transmitter module, one or more transmitter extender modules and an end terminator module are plugged together in series, being held in a channel member having a U-shaped cross-section, the arms of the U having inturned feet which fit into grooves in the sides of the modules. The arms are flexible to permit the number of transmitter extender modules to be changed. A similar arrangement applies at a receiver, a receiver module replacing the transmitter module and receiver extender modules replacing the transmitter extender modules. The transmitter module has on its face, 8 pairs of input terminals to be connected to switches or other on/off apparatuses to be monitored, and a pair of terminals for connection by a transmission line pair (or radio can be used), to a pair of terminals on the face of the receiver module which also bears 8 pairs of output terminals for connection to lamps or aural warning devices. Each transmitter extender module and receiver extender module has 16 pairs of terminals for the same purposes as the 8 on the transmitter module and receiver module respectively. The transmitter module includes an oscillator driving a binary counter via frequency dividers, a control flip-flop and 8 NAND gates. The first 8 counts are used to transmit a neutral period, the next 8 being recognized by respective ones of the NAND gates to transmit the states of respective ones of the devices connected to the input terminals. Each transmitter extender module includes a control flip-flop, and 16 NAND gates responsive to the count in the transmitter module to transmit the states of respective ones of the 16 inputs. The various control flip-flops are switched on in turn by the counter to enable the respective modules in turn to transmit their input information as described above. When the last transmitter extender has transmitted, the end terminator module returns a signal to reset the transmitter. A diode bridge is connected across the two transmission lines to prevent overload. At the receiver, the signal on the transmission line-pair is subjected to commonmode noise rejection in a circuit using two operational amplifiers and a resistor network, the circuit also including Zener diodes for overload protection. This circuit is in the receiver module and produces a pulse on one of two leads for each input bit. Control flip-flops, one in the receiver module, and two in each receiver extender module, are set in turn to gate the signals on the two leads to respective sets of 8 clocked flip-flops in the modules, by a 4-bit binary counter in the receiver module driven via a one-shot by the OR of the two leads. The lowest 3 outputs of this counter are decoded in the receiver module and each receiver extender module to select the flip-flops in the selected set in turn to receive the data. Each pulse at the input to the one-shot is also used to discharge a capacitor in a neutral period detector, this capacitor being charged from a voltage source via a resistor. During the neutral period before the next message, the capacitor is permitted to charge sufficiently to activate a pair of transistors to pass the contents of all the sets of 8 flip-flops to the output terminals of the receiver and receiver extender modules. In the receiver module, a second 4-bit binary counter is driven at about 16 times the speed of the other counter by an oscillator via frequency dividers. Each pulse from the one-shot mentioned (feeding the first counter in the receiver module), resets the second counter and the dividers. A particular count is recognized by a gate to produce a short pulse which is combined with the inverse output of the one-shot in a NAND gate, any change in the output of the NAND gate indicating a noise pulse and resetting the whole receiver, the message so far being lost.